The present invention relates to the measurement and monitoring of fluid flow, using optical fibres.
The hot wire anemometer has been long known as a device for measuring fluid flow [1]. The device comprises a thin wire typically a few millimeters long which is placed orthogonally to the direction of the flow to be measured. A voltage is applied across the wire, and the resulting current flow through the wire, which is inversely proportional to the resistance according to Ohm's law, heats the wire. This heat is transferred to the fluid at a rate which is dependent on the flow velocity of the fluid, until a final equilibrium temperature of the wire is reached. The wire resistance depends on its temperature, so the current can be related to the flow velocity. Measurement of the current can therefore be used to calculate the flow rate.
The principle of the hot wire anemometer depends on the cooling effect of a fluid flowing past a heated object, so that a measurement of the temperature of the object, or a parameter related thereto, gives an indication of the rate of flow.
It is frequently useful to obtain measurements of the rate of flow of oil, gas and water within the well bore of an oil well. The hot wire anemometer is disadvantageous in this situation. The device provides a single localised measurement, where well bores can be hundreds or thousands of meters deep, with the fluid flow at all or many depths being of interest. Further, it is preferred not to use electric current in oil industry sensors because of the risk of explosion. Also, the thin wire is fragile, and hence unsuited to the rigours of the downhole environment.
In contrast, optical fibres are known to be well-suited for downhole sensing applications. They are robust enough to withstand the high temperatures and pressures, and operate without electric current. Distributed measurements can be obtained representing the whole length of a fibre, thus providing a more complete picture than individual discrete measurements. In particular, optical fibres have been shown to be of use for downhole temperature sensing. One technique detects changes in backscattered light from within the fibre caused by changes in temperature.
Consequently, a number of flow measurement methods have been proposed which exploit the proven technology of optical fibre temperature sensing. A first technique [2] uses the cooling effect of flowing fluid exploited in the hot wire anemometer. A heater cable is disposed within a well bore, and is heated by current being passed through it. Optical fibres are arranged adjacent to the cable, and operate as temperature sensors to measure the temperature of the heated cable as it is cooled by flowing oil. This gives a temperature profile over the extent of the cable, from which the fluid flow is determined. Thus a distributed measurement is possible, but the cable-fibre structure is bulky and awkward to fabricate, in part because of the need for adequate electrical insulation. Also, the accuracy of the measurements relies on providing good thermal contact between the cable and the fibres.
A further technique relies on the transfer of heat from a heat source to the fluid [3]. A thermal sensor, which may have the form of an optical fibre, is arranged downhole adjacent to a thermal source. The source is heated, and the sensor is used to measure changes in the fluid caused by the transfer of heat to the fluid. The flow rate is calculated from the amount of heat transferred. This two-part arrangement of sensor and source is complex to deploy, operate and maintain, and the results require the distance between source and sensor to be considered.
A similar arrangement of equipment is used in a more recent approach which, however, is less mathematically complex [4]. A fibre temperature sensor is deployed in the well bore together with a heat exchanger arranged upstream with respect to the direction of oil flow. A quantity of oil is heated or cooled by the heat exchanger, and the temperature sensor detects the presence of this oil at two or more positions as it flows up the bore. From this, the flow velocity is calculated. This is a simple approach, but requires the heat exchanger to be arranged in the passage of the oil, which then can disturb the flow.
An alternative method relies in the long term only on a fibre temperature sensor, with no other downhole equipment being required [5]. However, a first step in the method uses dedicated conventional flow measurement means such as spinner or Venturi methods to obtain localised flow measurements, at the same time as measuring the temperature profile of the well using the fibre. This provides a calibration relating temperature to flow from which a model is derived. Subsequently, the model is used to calculate flow from future temperature measurements. The method is mathematically intensive, because many parameters describing the well are required to obtain an accurate model.
Hence there is a requirement for an improved method of monitoring flow, using simple apparatus.